Asymptomatic Spread of COVID-19 May be Less Common than We Thought

COVID-19 spread by asymptomatic carriers appears to be rare, according to WHO technical lead for coronavirus response Maria Van Kerkhove, speaking during a media briefing on Monday:

From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual. We have a number of reports from countries who are doing very detailed contact tracing. They’re following asymptomatic cases, they’re following contacts and they’re not finding secondary transmission onward. It is very rare — and much of that is not published in the literature. We are constantly looking at this data and we’re trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward.

Van Kerkhove also said that many cases that appear to be asymptomatic actually turn out to be mildly symptomatic. While some cases are truly asymptomatic, the weight of evidence seems to suggest that transmission by these individuals is rare.

This may seem at odds with previous reports concerning asymptomatic transmission (and did cause some confusion following the briefing), but it is important to make the distinction between asymptomatic and pre-symptomatic cases. Asymptomatic carriers are those who do not experience any symptoms for the duration of their infection, but any individual who does experience symptoms may still be infectious for two to three days before their symptoms begin, according to a study published in April. Some individuals can also develop mild or unusual symptoms and therefore do not realise that they have the virus.

Transmission of the virus by people who are not feeling unwell is therefore still a problem – but perhaps less of a problem than originally thought.